Performance requirements for a code division multiple access (CDMA) cellular subscriber mobile station are specified in Electronic Industries Association EIA/TIA/IS-95 "Mobile Station--Land Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System", published July 1993 (herein referred to as "IS-95 Standard"). The IS-95 Standard specifies a minimum dynamic range for output power control of a transmit signal and a minimum amount of permitted transmit sideband noise emissions.
The minimum dynamic range for output power control specified for a class III mobile station is 73 dB (-50 dBm to +23 dBm). When transmit gain tolerances are considered, the required dynamic range is 85 dB.
The transmit sideband emissions specification calls out a dBc limit which is applicable at higher output power and an emission floor which is applicable at lower output power levels. For frequency offsets from the carrier frequency between 900 kHz and 1.98 MHz, the maximum emission must be less than the greater of 42 dBc/30 kHz relative to the desired transmit power in a 1.23 MHz bandwidth or both -60 dBm/30 kHz and -55 dBm/1 MHz. For frequency offsets from the carrier greater than 1.98 MHz, the maximum emission must be less than the greater of -54 dBc/30 kHz relative to the desired transmit power in a 1.23 MHz bandwidth or both -60 dBm/30 kHz and -55 dBm/1 Mhz. To produce high quality mobile stations, 10 dB of margin is added to the sideband emission specification. Therefore, the design target for the emissions floor (-60 dBm/30 kHz and -55 dBm/1 MHz) is -70 dBm/30 kHz and -65 dBm/1 MHz.
In other cellular systems (AMPS, NAMPS, NADC, GSM, PDC, etc.) the dynamic range for output power control required for mobile stations is typically much lower (i.e. 20 to 30 dB) than the dynamic range for output power control required (i.e. 85 dB) for CDMA mobile stations. In these other systems, the required dynamic range for output power control is typically provided by controlling a variable gain stage, such as a variable gain power amplifier (PA), which amplifies a radio frequency (RF) signal or by controlling a voltage controlled attenuator (VCA) which attentuates an intermediate frequency (IF) signal. Individually, these schemes do meet the dynamic range requirement for output power control or the sideband emission requirement for CDMA mobile stations.
Good transmit sideband emission performance is obtained when the gain control circuitry for the RF signal is placed close to the antenna. Unfortunately, under this condition, it is not easy to realize 85 dB of gain control of the RF signal without providing very good shielding and grounding.
A gain control range of 85 dB can be realized at a transmit signal in the IF range which is typically 100 to 200 Mhz. However, controlling an 85 dB dynamic range of power control in the IF range is disadvantageous because it does not optimize the sideband noise emissions requirement. To meet the sideband noise emissions requirement, the gain following the gain control stage must be minimized in order to minimize the sideband noise produced in the transmitter at low output power levels. This requires a higher output level out of the transmit IF gain stages. This implies high linearity for the transmit IF gain stages which results in higher current drain. For example, the SONY CXA3002N transmit gain control amplifier has 85 dB of dynamic range at intermediate frequencies only, a +10 dBm output third order intercept point (OIP3), and a current drain of 35 mA.
Another disadvantage of having the 85 dB gain control stage control the transmit signal in the IF range is the susceptibility to spurs and noise generated in other sections of the radio. For example, if the maximum output power out of the gain controlled stage is -5 dBm for adequate linearity and the worst case maximum gain following the gain controlled stage is 35 dB, the maximum noise and spurs picked up at this point must be less than both -105 dBm/30 kHz and -90 dBm/1 MHz to pass the emission floor with good margin. It is not impossible to achieve these levels, however, this would probably require the use of extra shielding and several board and/or IC revisions. Even if this degree of isolation is achieved, the current drain would still be higher than desired.
Accordingly, there is a need for a power level control circuit for a RF transmitter which provides a wide dynamic range for output power control while minimizing the sideband noise emissions, the current drain, and the complexity of the RF transmitter.